Optical communications systems, that is systems operating in the visible or near visible spectra, are now at an advanced state of development. Such systems utilize glass fibers as the transmission medium. These fibers, generally having an overall cross-sectional diameter of about 125 .mu.m, are generally composed of at least two portions, a central core and a cladding layer disposed about the core. The cladding layer has an index of refraction which is less than that of the core, with a typical index variation from the core to the cladding layer being in the range from about 0.01 to 0.05. Optical fibers in use may be single-mode or multimode. The former is characterized by a sufficiently small core to accommodate efficiently only the first order mode. Such single-mode optical structures may have a core diameter of about 8 .mu.m. Multimode optical fibers typically have cores which have a diameter in the range of about 50 .mu.m to 100 .mu.m.
In the manufacture of optical fiber, a glass preform rod which generally is manufactured in a separate process is suspended vertically and moved into a furnace at a controlled rate. The preform rod softens in the furnace and optical fiber is drawn freely from a molten end of the preform rod by a capstan located at the base of a draw tower. Because the surface of the optical fiber is very susceptible to damage caused by abrasion, it becomes necessary to coat the optical fiber, after it is drawn, but before it comes into contact with any surface.
One of the most important parameters for an optical fiber is loss. Loss which is expressed in decibels per kilometer (dB/Km) can be caused by absorption of impurities in the glass, or by scattering of the light. Production optical fibers are made of glass which contains impurities, as well as compositional variations. Optical loss can be plotted as a function of the wavelength of light passing through the optical fiber. For silica fibers doped with germanium and/or phosphorus, there is a minimum loss at a wavelength of about 1.55 .mu.m. The selection of doped silica glass takes advantage of the "window" between infrared molecular vibration absorption and Rayleigh scattering plus ultraviolet absorption regions or "tails".
Rayleigh scattering results from density and compositional variations within the fiber material. Rayleigh scattered energy is absorbed in the cladding and guided in a backward direction. During the processing of optical fiber, it is desired to reduce the impurity absorption loss to zero so that only the loss due to Rayleigh scattering remains. However, other forms of scatter loss may occur and impede the achievement of this goal. One is loss caused by variations in the size of the fiber core. Dimensional variations introduced into production fiber can cause loss by light scattering and influence the quality of a fiber connector or splice because of size differences.
Absorption results when light photons contain sufficient energy to excite electrons of the glass constituent materials. In transparent media of pure silica, the oxygen atoms have very tightly bound electrons and only ultraviolet light photons have enough energy to be absorbed. However, the silica in optical fiber includes dopants and transition-metal impurities whose electrons can be excited by lower energy light. Those constituents cause the ultraviolet absorption to be shifted lower and cause additional absorption bands in the visible and near infrared ranges. The amount of loss which is caused by the presence of these impurities depends on their concentration. At certain wavelengths, relatively small concentrations of impurities may cause an increase in absorption loss of about 1 dB/Km.
In addition to the problem of impurity absorption, another problem arises from the presence of the hydroxyl (OH.sup.-) ion. The fundamental vibration of this ion occurs at a wavelength of 2.7 .mu.m, but overtones at 0.95 and 1.4 .mu.m extend to the priorly mentioned window region. Concentrations of OH.sup.- as low as one part per million can cause losses as high as 1 dB/Km at 0.95 .mu.m and 50 dB/Km at 1.4 .mu.m. Obviously, it is important to reduce these so-called water peaks to as low a value as possible to achieve the lowest loss in transmission windows around 1.3 .mu.m and 1.55 .mu.m.
Low loss silica-rich fibers are made by using silicon tetrachloride (SiCl.sub.4) as a precursor material. This material typically is very low in transition-metal ion impurities, but may contain substantial amounts of hydrogen-bearing compounds. Because the glass for the fiber is formed in a process by reacting the SiCl.sub.4 with oxygen, OH.sup.- can form readily if hydrogen is present. Hence, it is important not only to use precursor materials that are low in hydrogen content, but also to prevent entry of similar compositions into the process or inadvertent contamination by handling.
Optical fiber loss also has been found to depend on conditions during the drawing of the fiber from a preform such as, for example, the temperature of the preform during draw, and the speed at which the fiber is drawn. These last two mentioned conditions also determine the fiber tension during draw. During the drawing of the optical fiber, defects are generated and the number of defects is dependent on the drawing conditions which of course can cause changes in the loss. However, these defects seemingly are rendered benign if the preform from which the optical fiber is being drawn includes alkalies.
Although the presence of alkalies is helpful in rendering impotent those defects caused during drawing, another problem concerning absorption losses revolves around the use of precursor tubes, the composition of which includes relatively low levels, i.e. about 100 to 0.1 ppm atomic, of alkalies. It has been found that the long term hydrogen effect attributed to the presence of alkalies affects adversely the performance of the optical fiber drawn from a preform which has been manufactured from such a precursor tube. This problem becomes particularly acute when using a so-called rod and tube process in which a preform rod is overclad with a tube to provide a larger preform for draw. During the processing of a substrate tube which includes alkalies in a normal MCVD process, for example, the alkali level somehow is reduced. In the rod and tube process, which is today becoming very popular as a means to save costs, this does not occur and long term hydrogen-related absorption losses will increase.
Because of such adverse effects, efforts have been made to provide precursor quartz tubes which are substantially alkali-free. It has been found that absorption losses which have been somewhat troublesome in the past may increase substantially when using alkali-free precursor tubes.
What were needed were methods and apparatus which provide an optical fiber having relatively low drawing-induced absorption losses. A method of making an optical fiber having a relatively low absorption loss includes the step of moving an optical preform incrementally into a device in which the preform is exposed to heat energy and then drawing optical fiber from each successive portion of the preform as it is exposed to the heat energy. Then the temperature of the drawn fiber is caused to decrease in a controlled manner which causes the absorption loss of the drawn fiber to be relatively low. In a preferred embodiment, the temperature of the drawn optical fiber is caused to decrease gradually to avoid an abrupt change in that parameter which has been commonplace in the prior art. This may be accomplished, for example, by appending a recovery tube to a lower end of a draw furnace or by engaging the drawn optical fiber with a gas at a controlled temperature or both to anneal the drawn fiber in a controlled manner.
It should be apparent that the use of a recovery tube to anneal defects is line speed limited and that to achieve the best results, the line speed needs to be on the low end of the scale. This, of course, is counterproductive to recent efforts to increase line speed. Higher line speeds are achievable because of improved optical fiber coating techniques, but the annealing step remains line speed limiting.
What is sought after and seemingly what is not available in the prior art are methods and apparatus for annealing drawn optical fiber to reduce loss without limiting the speed at which the optical fiber is moved along a drawing line. Such sought after methods and apparatus should be capable of being integrated easily into existing optical fiber draw lines or towers as they commonly are called.